1. Field
The present disclosure is directed to a method and apparatus for channel feedback in a wireless system with antenna arrays. More particularly, the present disclosure is directed to feedback from a wireless terminal to a base station.
2. Introduction
A Multi-Input Multi-Output (MIMO) communication system uses a plurality of channels in a spatial area. Antenna arrays that have multiple transmission antennas can increase the capacity of data transmission through MIMO transmission schemes.
Two types of MIMO transmission schemes can be employed. Single-user MIMO (SU-MIMO) involves transmitting at least one data stream to a single user over a time frequency resource. Multi-user MIMO (MU-MIMO) involves transmitting at least one data stream per user to at least two co-scheduled users over a single, i.e., same time frequency, resource.
In a MIMO communication system, base stations and mobile stations use codebooks to enable channel state information feedback. The codebooks may additionally be used for precoding the information streams at the transmitter. Elements of a codebook are stored in the base stations and mobile stations and can be used to quantize the spatial channel state information for feedback. Each codebook element may be a vector or a matrix depending on the dimension of a channel matrix and the number of data streams that can be supported. Each mobile station selects one matrix or vector corresponding to channel information from among matrices or vectors included in the codebook according to a channel formed between the base station and the mobile station. Each base station uses a codebook to recognize channel information by receiving the matrix or vector selected by each mobile station. The selected matrix or vector may be used for beamforming or, more generally, precoding the one or more data streams prior to transmission by the base station using multiple antennas. Precoding is used to emit multiple data streams from the antenna array appropriate weightings such that the link throughput is maximized.
Typically, MIMO systems support a maximum of eight Channel State Information Reference Signal (CSI-RS) ports for determining channel state information feedback. However, base station antenna arrays may employ more than eight antenna elements, which exceed the number of CSI-RS antenna ports available for computing feedback at the wireless terminal. Furthermore, large antenna arrays may require additional Channel State Information (CSI) and precoding matrix signaling.